Dual Access Pathways to Serially-Connected Mass Data Storage Units

1. In a mass data storage system, an improvement comprising: a first data storage unit to which data storage operations are communicated, the first data storage unit including at least one data storage device which stores data, the first data storage unit including first and second interfaces to communicate the data storage operations to each data storage device of the first data storage unit; a second data storage unit to which data storage operations are communicated, the second data storage unit including at least one data storage device which stores data, the second data storage unit including first and second interfaces to communicate the data storage operations to each data storage device of the second data storage unit; a first filer which manages data storage operations of the first data storage unit, the first filer including first and second adapters for operatively communicating the data storage operations to the interfaces of the first and second data storage units; a second filer which manages data storage operations of the second data storage unit, the second filer including first and second adapters for operatively communicating the data storage operations to the interfaces of the first and second data storage units; a first main primary communication pathway connector extending from the first adapter of the first filer to the first interface of the first data storage unit; a first redundant primary communication pathway connector extending from the first adapter of the first filer to the second interface of the first data storage unit; a second main secondary communication pathway connector extending from the second adapter of the second filer to one of the first or second interfaces of the first data storage unit; a second redundant secondary communication pathway connector extending from the second adapter of the second filer to the one of the first or second interfaces of the first data storage unit other than the one interface to which the second main secondary communication pathway connector of the second filer is connected; a second main primary communication pathway connector extending from the first adapter of the second filer to the first interface of the second data storage unit; a second redundant primary communication pathway connector extending from the first adapter of the second filer to the second interface of the second data storage unit; a first main secondary communication pathway connector extending from the second adapter of the first filer to one of the first or second interfaces of the second data storage unit; and a first redundant secondary communication pathway connector extending from the second adapter of the first filer to the one of the first or second interfaces of the second data storage unit other than the one interface to which the first main secondary communication pathway connector from the first filer is connected.

2. In a mass data storage system, an improvement comprising: a group of data storage units which are serially connected in a sequential data communication path in which data storage operations are communicated, each data storage unit including at least one data storage device which stores data, each data storage unit including first and second interfaces to communicate the data storage operations to each data storage device of the group; a first filer which manages data storage operations of the data storage units in the group, the first filer including an adapter for operatively communicating the data storage operations to the interfaces of the data storage units in the group; a second filer which also manages data storage operations of data storage units in the group, the second filer including an adapter for operatively communicating the data storage operations to the interfaces of the data storage units in the group; a main primary communication pathway connector extending from the adapter of the first filer to the first interface of one data storage unit in the group; a redundant primary communication pathway connector extending from the adapter of the first filer to one of the first or second interfaces of a data storage unit in the group other than the interface to which the main primary communication pathway connector is connected; a main secondary communication pathway connector extending from the adapter of the second filer to the second interface of one data storage unit in the group; and a redundant secondary communication pathway connector extending from the adapter of the second filer to one of the first or second interfaces of a data storage unit in the group other than the second interface to which the main secondary communication pathway connector is connected.

3. In a mass data storage system as defined in claim 1 , further comprising: a first group of a plurality of the data storage units, the first group including the first data storage unit; and a second group of a plurality of the data storage units, the second group including the second data storage unit; and wherein: the first main and first redundant primary communication pathway connectors connect to different interfaces in the first group; the first main and first redundant secondary communication pathway connectors connect to different interfaces in the second group; the second main and second redundant primary communication pathway connectors connect to different interfaces in the second group; and the second main and second redundant secondary communication pathway connectors connect to different interfaces in the first group.

4. In a mass data storage system as defined in claim 1 , further comprising: a first group of a plurality of the data storage units, the first group including the first data storage unit; and a second group of a plurality of the data storage units, the second group including the second data storage unit; and wherein: the first main primary and second main secondary communication pathway connectors connect to different interfaces of the same data storage unit in the first group; and the first redundant primary and second redundant secondary communication pathway connectors connect to different interfaces of the same data storage unit in the first group.

5. In a mass data storage system as defined in claim 1 , further comprising: a first group of a plurality of the data storage units, the first group including the first data storage unit; and a second group of a plurality of the data storage units, the second group including the second data storage unit; and wherein: the first and second main primary communication pathway connectors connect to first interfaces of the data storage units in the first and second groups, respectively; and the first and second redundant secondary communication pathway connectors connect to the second interfaces of the data storage units in the second and first groups, respectively.

6. In a mass data storage system as defined in claim 3 , wherein: the main and redundant primary communication pathway connectors connect to the first interfaces of different ones of the data storage units in the group.

7. In a mass data storage system as defined in claim 6 , wherein: the sequential data communication path connecting the data storage units in the group commences with a beginning data storage unit and ends with an ending data storage unit in the group; the main primary communication pathway connector connects to the first interface of one of the beginning or ending data storage units in the group; and the redundant primary communication pathway connector connects to the first interface of the other one of the beginning or ending data storage units in the group.

8. In a mass data storage system as defined in claim 3 , wherein: the main primary communication pathway connector connects to the first interface of one of the data storage units in the group; and the redundant primary communication pathway connector connects to the second interface of one of the data storage units in the group.

9. In a mass data storage system as defined in claim 8 , wherein: the main and redundant primary communication pathway connectors are connected to the first and second interfaces of different ones of the data storage units in the group.

10. In a mass data storage system as defined in claim 9 , wherein: the sequential data communication path connecting the data storage units in the group commences with a beginning data storage unit and ends with an ending data storage unit in the group; the main primary communication pathway connector connects to the first interface of one of the beginning or ending data storage units in the group; and the redundant primary communication pathway connector connects to the second interface of the other one of the beginning or ending data storage units in the group.

11. In a mass data storage system as defined in claim 1 , wherein: the first and second filers are connected together to communicate commands to establish one of the first and second filers to manage data storage operations of the first data storage unit; the first filer communicates signals with the first data storage unit over the first main and first redundant primary communication pathway connectors to determine the integrity of the communication paths to the first data storage unit; and the first filer commands the second filer to assume management of data storage operations of the first data storage unit upon determining that the communication path to the first data storage unit through neither of the first main and first redundant primary communication pathway connectors exists.

12. In a mass data storage system as defined in claim 2 , wherein: the main and redundant secondary communication pathway connectors connect to the second interfaces of different ones of the data storage units in the group.

13. In a mass data storage system as defined in claim 12 , wherein: the sequential data communication path connecting the data storage units in the group commences with a beginning data storage unit and ends with an ending data storage unit in the group; the main secondary communication pathway connector connects to the second interface of one of the beginning or ending data storage units in the group; and the redundant secondary communication pathway connector connects to the second interface of the other one of the beginning or ending data storage units in the group.

14. In a mass data storage system as defined in claim 2 , wherein: the main secondary communication pathway connector connects to the second interface of one of the data storage units in the group; and the redundant secondary communication pathway connector connects to the first interface of one of the data storage units in the group.

15. In a mass data storage system as defined in claim 14 , wherein: the main and redundant secondary communication pathway connectors are connected to the second and first interfaces of different ones of the data storage units in the group.

16. In a mass data storage system as defined in claim 15 , wherein: the sequential data communication path connecting the data storage units in the group commences with a beginning data storage unit and ends with an ending data storage unit in the group; the main secondary communication pathway connector connects to the second interface of one of the beginning or ending data storage units in the group; and the redundant secondary communication pathway connector connects to the first interface of the other one of the beginning or ending data storage units in the group.

17. In a mass data storage system, an improvement comprising: a group of data storage units which are serially connected in a sequential data communication path in which data storage operations are communicated, each data storage unit including at least one data storage device which stores data, each data storage unit including first and second interfaces to communicate the data storage operations to each data storage device of the group; a first filer which is operative to manage data storage operations of the data storage units in the group, the first filer including an adapter for operatively communicating the data storage operations to the interfaces of the data storage units in the group; a second filer which is also operative to manage data storage operations of the data storage units in the group, the second filer including an adapter for operatively communicating the data storage operations to the interfaces of the data storage units in the group; a main primary communication pathway connector extending from the adapter of the first filer to the first interface of one data storage unit in the group; a redundant primary communication pathway connector extending from the adapter of the first filer to one of the first or second interfaces of a data storage unit in the group other than the interface to which the main primary communication pathway connector is connected; a main secondary communication pathway connector extending from the adapter of the second filer to the second interface of one data storage unit in the group; and wherein: the first and second filers are connected to communicate commands to establish one of the first and second filers to manage data storage operations of the data storage units in the group; the first filer communicates signals with each data storage unit in the group over the main and redundant primary communication pathway connectors to determine the integrity of the sequential data communication path to each data storage unit of the group through the main and redundant primary communication pathway connectors; and the first filer commands the second filer to assume management of data storage operations of the data storage units in the group upon determining that the sequential data communication path to each data storage unit of the group through neither of the main and redundant primary communication pathway connectors exists.

18. In a mass data storage system as defined in claim 17 , wherein: the first filer commands the second filer to assume management of data storage operations only upon further determining that a volume of data written to the data storage devices of the group can not be reconstructed from data storage operations performed to the data storage devices of the group that remain accessible.

19. In a mass data storage system as defined in claim 2 , wherein: the aforesaid plurality of data storage units constituting a first group of data storage units; the aforesaid main primary communication pathway connector extending from the adapter of the first filer to the first interface of one data storage unit in the first group constituting a first main primary communication pathway connector; the aforesaid redundant primary communication pathway connector extending from the adapter of the first filer to one of the first or second interfaces of a data storage unit in the first group other than the interface to which the first main primary communication pathway connector is connected constituting a first redundant primary communication pathway connector; the aforesaid main secondary communication pathway connector extending from the adapter of the second filer to the second interface of one data storage unit in the first group constituting a second main secondary communication pathway connector; and the aforesaid redundant secondary communication pathway connector extending from the adapter of the second filer to one of the first or second interfaces of a data storage unit in the first group other than the second interface to which the second main secondary communication pathway connector is connected constitutes a second redundant secondary communication pathway connector; and further comprising: a second group of data storage units which are serially connected in a sequential data communication path in which data storage operations are communicated, each data storage unit in the second group including at least one data storage device which stores data, each data storage unit in the second group including first and second interfaces to communicate the data storage operations to each data storage device of the data storage unit in the second group; a second main primary communication pathway connector extending from the adapter of the second filer to the first interface of one data storage unit in the second group; a second redundant primary communication pathway connector extending from the adapter of the second filer to one of the first or second interfaces of a data storage unit in the second group other than the interface to which the second main primary communication pathway connector is connected; a first main secondary communication pathway connector extending from the adapter of the first filer to one of the first or second interfaces of a data storage unit in the second group other than the interface to which the second main primary communication pathway connector is connected; and a first redundant secondary communication pathway connector extending from the adapter of the first filer to one of the first or second interfaces of a data storage unit in the second group other than the interface to which the first main secondary communication pathway connector is connected.

20. In a mass data storage system as defined in claim 19 , wherein: the second main and second redundant secondary communication pathway connectors connect to the second interfaces of different ones of the data storage units of the first group.

21. In a mass data storage system as defined in claim 20 , wherein: the sequential data communication path connecting the data storage units in each group commences with a beginning data storage unit and ends with an ending data storage unit in each group; the first and second main secondary communication pathway connectors connect to the second interfaces of one of the beginning or ending data storage units in each group; and the first and second redundant secondary communication pathway connectors connect to the second interfaces of the other one of the beginning or ending data storage units in each group.

22. In a mass data storage system as defined in claim 19 , wherein: the first and second main secondary communication pathway connectors connect to the second interface of one of the data storage units in each group; and the first and second redundant secondary communication pathway connectors connect to the first interface of one of the data storage units in each group.

23. In a mass data storage system as defined in claim 19 , wherein: the first and second main and redundant secondary communication pathway connectors are connected to the second and first interfaces of different ones of the data storage units within each group.

24. In a mass data storage system as defined in claim 23 , wherein: the sequential data communication path connecting the data storage units in each group commences with a beginning data storage unit and ends with a ending data storage unit in each group; the first and second main secondary communication pathway connectors connect to the second interface of one of the beginning or ending data storage units in each group; and the first and second redundant secondary communication pathway connectors connect to the first interface of the other one of the beginning or ending data storage units in each group.

25. In a mass data storage system as defined in claim 19 , wherein: the first and second filers are connected to communicate commands to establish the one of the first and second filers to manage data storage operations of the data storage units in the first and second groups; the first filer communicates signals with each data storage unit in the first group over the first main and first redundant primary communication pathway connectors to determine the integrity of the sequential data communication path to each data storage unit of the first group through the first main and first redundant primary communication pathway connectors; the second filer communicates signals with each data storage unit in the second group over the second main and second redundant primary communication pathway connectors to determine the integrity of the sequential data communication path to each data storage unit of the second group through the second main and second redundant primary communication pathway connectors; the first filer commands the second filer to assume management of data storage operations of the data storage units in the first group upon determining that the sequential data communication path to each data storage unit of the first group through neither of the first main and first redundant primary communication pathway exists; and the second filer commands the first filer to assume management of data storage operations of the data storage units in the second group upon determining that the sequential data communication path to each data storage unit of the second group through neither of the second main and second redundant primary communication pathway exists.

26. In a mass data storage system as defined in claim 25 , wherein: the first filer commands the second filer to assume management of data storage operations of the data storage units of the first group upon further determining that a volume of data written to the data storage devices of the first group can not be reconstructed from data storage operations performed to the data storage devices of the first group that remain accessible; and the second filer commands the first filer to assume management of data storage operations of the data storage units of the second group upon further determining that a volume of data written to the data storage devices of the second group can not be reconstructed from data storage operations performed to the data storage devices of the second group that remain accessible.

27. A method for enhancing redundancy in communication between at least one data storage unit in a group and a first filer and a second filer in a mass storage system, comprising: creating a first serial communication pathway from the first filer with each data storage unit in the group; creating a second serial communication pathway from the first filer with each data storage unit in the group, the second serial communication pathway having at least a portion which is different from the first serial communication pathway; communicating data storage operations over the first serial communication pathway between the first filer and all of the data storage units in the group; detecting a failure in communicating the data storage operations over the first serial communication pathway between the first filer and any data storage unit in the group, and thereafter communicating data storage operations over the second communication pathway between the first filer and the data storage units in the group; creating a third serial communication pathway from the second filer with each data storage unit in the group, the third serial communication pathway having at least a portion which is different from the first and second serial communication pathways; detecting a failure in communicating the data storage operations over the second serial communication pathway between the first filer and any data storage unit in the group, and thereafter communicating data storage operations over the third serial communication pathway between the second filer and the data storage units in the group.

28. A method as defined in claim 27 , further comprising: creating a fourth serial communication pathway from the second filer with each data storage unit in the group, the fourth serial communication pathway having at least a portion which is different from the first, second and third serial communication pathways; detecting a failure in communicating the data storage operations over the third serial communication pathway between the second filer and any data storage unit in the group, and thereafter communicating data storage operations over the fourth serial communication pathway between the second filer and the data storage units in the group.

29. A method as defined in claim 27 , further comprising: including a plurality of data storage units in the serial communication path in the group; storing data in a volume on a plurality of the data storage units in the group; attempting to reconstruct data which has become inaccessible due to the failure of the first and second serial communication pathways by accessing other data storage units that remain accessible to the first filer through the first and second serial communication pathways before communicating the data storage operations over the third serial communication pathway between the second filer and the data storage units in the group; and continuing to communicate the data storage operations over the first and second serial communication pathways without communicating the data storage operations over the third serial communication pathway so long as no data in a volume becomes inaccessible.

30. A method as defined in claim 29 , further comprising: including a plurality of data storage units in the serial communication path in the group; including at least one data storage device in each data storage unit, each data storage device storing data; including a pair of interfaces in each data storage unit which each communicate the data storage operations to each data storage device of each data storage unit; serially linking each of the first ones of the interfaces of all data storage units in the group as part of at least one communication pathway; serially linking each of the second ones of the interfaces of all the data storage units in the group as part of at least one communication pathway; principally communicating the data storage operations to the first interfaces of the data storage units in the group; and connecting the first serial communication pathway to one of the first interfaces.

31. A method as defined in claim 30 , further comprising: connecting the second serial communication pathway to one of the first interfaces which is different from the first interface to which the first serial communication pathway is connected.

32. A method as defined in claim 30 , wherein the serially linked data storage units in the group commence with a beginning data storage unit and end with an ending data storage unit in communication sequence in the group, the method further comprising: connecting the first serial communication pathway to the first interface of one of the beginning or ending data storage units in the group; and connecting the second serial communication pathway to the first interface of the other one of the beginning or ending data storage units in the group.

33. A method as defined in claim 30 , further comprising: connecting the second serial communication pathway to a second interface of a data storage unit in the group.

34. A method as defined in claim 30 , further comprising: connecting the third serial communication pathway to one of the second interfaces of the data storage units in the group; detecting a failure in communicating the data storage operations over the second serial communication pathway between the first filer and any data storage unit in the group, and thereafter communicating data storage operations over the third serial communication pathway between the second filer and the data storage units in the group.

35. A method as defined in claim 34 , further comprising: creating a fourth serial communication pathway from the second filer with each data storage unit in the group, the fourth serial communication pathway having at least a portion which is different from the first, second and third serial communication pathways; connecting the fourth serial communication pathway to one of the interfaces which is different from the interface to which the third serial communication pathway is connected; detecting a failure in communicating the data storage operations over the third serial communication pathway between the second filer and any data storage unit in the group, and thereafter communicating data storage operations over the fourth serial communication pathway between the second filer and the data storage units in the group.

36. A method as defined in claim 35 , further comprising: connecting the fourth serial communication pathway to one of the second interfaces.

37. A method as defined in claim 35 , further comprising: connecting the fourth serial communication pathway to one of the first interfaces, a third serial communication pathway between a second filer and each of the data storage units, the third serial communication pathway having at least a portion different from the first and second communication pathways; wherein upon a failure of the first and second serial communications pathways, communicating a data storage operation over the third serial communication pathway between the second filer and one of the data storage units using the third serial communication pathway.

38. In a mass data storage system as defined in claim 3 , wherein: the first group of data storage units are serially connected in a sequential data communication path in which the data management operations are communicated; and the second group of data storage units are serially connected in a sequential data communication path in which the data management operations are communicated.

39. In a mass data storage system as defined in claim 4 , wherein: the first group of data storage units are serially connected in a sequential data communication path in which the data management operations are communicated; and the second group of data storage units are serially connected in a sequential data communication path in which the data management operations are communicated.

40. In a mass data storage system as defined in claim 5 , wherein: the first group of data storage units are serially connected in a sequential data communication path in which the data management operations are communicated; and the second group of data storage units are serially connected in a sequential data communication path in which the data management operations are communicated.